ESC energy recovery system for fuel-efficient aircraft

ABSTRACT

The invention is an energy recovery system (20) which uses excess air emanating from an aircraft cabin (16) to drive a turbo-compressor unit (30). The outlet air from the compressor (26) of turbo-compressor unit (30) is optionally fed, either around or through heat exchanger (24), back into the aircraft cabin (16) inlet line, reducing the amount of bleed air required to power the aircraft ECS. Mass flow sensors (34) and (38) are used, via flow control valve (36) and flow control valve control unit (40), to monitor and control the amount of engine bleed air required at any point in time to satisfy overall ECS requirements. The energy recovery system (20&#34;) may be utilized in conjunction with a motor driven cabin compressor (42) to reduce the output air requirements of the compressor and thus the weight and horsepower requirements of the compressor motor. In another embodiment, the outlet of the compressor (26) is used to supercharge the inlet of the motor driven cabin compressor (42), reducing the weight and horsepower motor requirements of the ECS.

TECHNICAL FIELD

The invention relates generally to the field of aircraft environmentalcontrol systems (ECS), and more particularly to an ECS energy recoverysystem which conserves energy, and thus reduces fuel consumption.

BACKGROUND ART

In many aircraft, the environmental control system, ECS, is powered fromengine "bleed air", i.e. air extracted from bleed-ports located at theintermediate and last stages of the aircraft engine compressors. This ispremium-air inasmuch as it impacts very unfavorably on the performanceof the engine, and results in thrust losses and fuel penalties on theengine. In the guest for fuel-efficient air-transports, it is essentialthat the method of extracting power for the air conditioning system, beoptimized from a power and fuel consumption point of view.

It is of importance and relevant to short-haul and short-to-mediumnrange aircraft that the same quest for fuel-conservation in theseaircraft could lead to the selection of turbo-prop engines. Theseengines, however, have lower "core-flows" than the jet engines and, as aconsequence, the amount of air available for cabin air conditioning isusually more limited. At the same time, these aircraft also have a highpassenger-density-to-aircraft-volume and therefore require good airventilations rates. On the other hand, many wide-bodied aircraft (whichhave lower passenger/volume densities), often dump large amounts ofcabin air overboard, in order to take on adequate quantities of freshair. This also reflects as a significant weight and fuel-penalty.

One prior art method of providing power to aircraft ECS systems is toutilize the aircraft engines to mechanically drive the cabincompressors, usually via a gearbox arrangement between the engine andthe compressor. Systems such as this are exemplified in U.S. Pat. Nos.2,614,815 to Marchant et al, 2,585,570 to Messinger et al, 2,678,542 toStanton, and 2,697,917 to Mayer.

While not using bleed air, these systems often involve nevertheless, useof complex gear drives, placement of compressors in hostile wingenvironments, and less than ideal fuel consumption characteristics.

Yet another method of driving cabin compressors is disclosed incopending U.S. patent applications U.S. Ser. No. 181,079, filed Sept. 2,l980, now abandoned for "Direct-Driven Generator System forEnvironmental Control System and Engine Starting", and U.S. Ser. No.183,609, filed Sept. 2, 1980, for "All-Electric Environmental ControlSystem For Advanced Transport Aircraft", both assigned to the assigneeherein. Both of these applications describe various types of ECS systemswhich are operated by an electric motor driven compressor. These systemsafford enhanced fuel conservation and several other advantages over thebleed air and engine-driven systems discussed hereinabove, and as suchare quite desirable.

Several other approaches to enhancing the efficiency of ECS systems havealso been attempted in the prior art. One such approah is disclosed inU.S. Pat. No. 3,711,044 to Matulich. The Matulich patent teachesutilizing an auxiliary gas turbine power unit to reduce the fuel demandof a conventional bleed air ECS by varying the speed of the compressorwhich supplies pressurized air to the ECS. This approach to increasingthe efficiency of the ECS system is based upon a complex control systemfor the integration of the ECS and the auxiliary power unit (APU) toprovide partial control of the APU in response to ECS requirements.

Another group of prior art patents teach the utilization, at least tosome extent, of aircraft cabin discharge air within an ECS systemostensibly designed for greater efficiency. U.S. Pat. No. 2,479,991 toWood, for example, discloses a primary cabin compressor driven by an APUwhich is mechanically driven, via an overrunning clutch, by twoturbines. Air from the primary compressor is cooled by expansion througha first turbine, which in turn, mechanically unloads the primarycompressor via the overrunning clutch. In addition, cabin air isdischarged through a second turbine which also augments the compressorvia the overruning clutch. This system involves mechanical energyfeedback, and is complex from a control standpoint. In addition, thesystem can only input mechanical energy back to the compressor when theturbines can drive through the "free running" clutch.

Other approaches to using cabin discharge air to increase ECS efficiencycan be found in U.S. Pat. Nos. 2,491,462 to Wood, 2,851,254 to Messingeret al, 2,777,301 to Kuhn, and 4,091,613 to Young. All of these patentsto some extent, utilize gas turbine power units (GTPU) interfaced withECS systems. These systems utilize several types of rotational elements,combustors and the like to superheat and expand cabin discharge air andput it back into the ECS. Such systems are typically complex andexpensive, and more importantly, are less than ideally fuel efficient inthat they use additional energy in the form of gas turbine fuel.

Yet another approach to utilizing cabin discharge air can be found inU.S. Pat. No. 3,369,777 to Furlong. Furlong teaches using cabindischarge air to drive an air turbine, which in turn drives a suctionfan that draws air through the double wall of the cabin. The dischargeair also drives a compressor that recompresses the discharge air beforeit is discharged overboard. The compressor is utilized to load theturbine. While this system recovers some of the energy of the cabindischarge air, it nevertheless is less than ideally efficient vis-a-visfuel comsumption and aircraft ECS operation.

While all of the systems of the above prior art United States patentsand copending applications, incorporated by reference herein, aredirected to obtaining various levels of efficiency in aircraft ECS andairflow systems, it is, nevertheless, desirable that aircraft ECSsystems be optimized in terms of efficiency in view of the present daystrong need for fuel-efficient air transports. It is essential, then,that the energy and fuel consumption in extracting power for aircraftECS be optimized (minimized), whether that power is extracted via engine"bleed air", "mechanical" or "electrical" power systems.

Thus, it is a primary object of this invention to provide a method andsystem for optimizing aircraft ECS systems in terms of energy, and thus,fuel consumption.

It is another object of this invention to provide an "energyutilization" or fresh-air "make-up" system for optimizing "bleed-air","mechanical", and "electrical" ECS systems in aircraft.

It is yet another object of this invention to provide a system forpressurizing-the-inlet of electrically or mechanically driven ECS cabincompressors to optimize such ECS systems in terms of fuel consumption.

It is another object of the present invention to provide a system forfeeding-back power to an electric motor which drives a cabin compressorin an ECS, to reduce the load on the motor and thereby conserve energy.

DISCLOSURE OF INVENTION

The invention comprises a system and process for the utilization ofwaste energy (in the form of aircraft cabin overboard discharge air) topower turbo machinery. The turbo machinery, in conjunctin with ram air,provides a fresh-air make-up system in the aircraft by supplyingpressurized fresh-air to the aircraft cabin during flight. This supplyof fresh-air significantly reduces the power demand for either bleed-airfrom the aircraft engines or drive shaft power for the cabin compressor.The turbo machinery comprises a turbo-compressor unit which is drivenduring flight by the differential in pressure between the cabin andoutside.

In another embodiment of the invention, a turbo-compressor unit,utilizing the energy of cabin discharge air, pressurizes the inlet to acabin compressor, thereby reducing its power demand on the electrical ormechanical drive source.

In yet another embodiment, the energy of cabin discharge air is utilizedto drive an air turbine motor mounted on the same shaft as a motor whichdrives the cabin compressor. The air turbine motor thus provides afeedback power contribution to the motor which drives the cabincompressor.

The novel features which are believed to be characteristic of theinvention, both as to its organization and to its method of operation,together with further objects and advantages thereof, will be betterunderstood from the following description considered in connection withthe accompanying drawings, in which presently preferred embodiments ofthe invention are illustrated by way of examples.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an "energy utilization""fresh-air make-up" system in accordance with a first embodiment of thisinvention, showing the system utilized in conjunction with a known"bleed-air" powered ECS system; FIG. 2 is a schematic representation ofa cabin compressor "pressurized inlet" system in accordance with asecond embodiment of this invention, showing the "energy utilization"system utilized in conjunction with an electrically driven cabincompressor; FIG. 3 is a schematic representation of a "fresh-airmake-up" system similar to the one depicted in FIG. 1, showing the"energy utilization" system utilized in conjunction with an electricallydriven cabin compressor; and FIG. 4 is a schematic representation of yetanother "energy utilization" system embodiment of the present inventionshowing excess cabin energy utilized to drive an air turbine motor whichprovides feedback power to the motor of a motor driven compressor unit.In all drawing Figures, like numerals denote like parts.

BEST MODE OF CARRYING OUT THE INVENTION

In accordance with the present invention, four novel embodiments of anaircraft ECS air conditioning system for optimization in terms of fuelconsumption are described. The methods involve the use of an energyrecovery system which, depending on whether the ECS is "bleed-air" ormechanically/electrically powered, operates as a "fresh-air make-up"system, a system for "pressurizing-the-inlet" of the cabin compressor ofthe ECs, or as a feedback power contribution system.

Referring now to FIG. 1, there is shown therein, a schematic of ableed-air driven ECS utilizing a "fresh-air make-up" "energy recovery"system in accordance with the present invention. Hot pressurizedbleed-air is taken from an engine (not shown) of a turbo-prop orturbo-jet/fan-jet aircraft via engine compressor taps and is cooled bythe primary heat exchanger (12). The thus cooled air is then passedthrough the expansion cooling system pack (14), which may include knownvapor-cycle or air-cycle cooling systems. Reference is made to copendingU.S. Ser. No. 183,079, now abandoned and U.S. Pat. No. 2,585,570, citedhereinabove, for disclosure of various air-cycle systems, includingaccessories such as compressors, turbines, heat-exchangers, and thelike, included therein. The conditioned and pressurized air emanatingfrom the ECS pack (14) is delivered to the aircraft cabin (16).

In large modern aircraft, such as the Lockheed L-1011, as much as about400 pounds per minute of air is supplied to the cabin (16) in order tomaintain the pressure and air quality desired, that is, to maintainpressurization and air supply. In such large aircraft, it is not unusualfor as much as about 220 ppm of this air to be discharged overboard.Since cabin leakage is normally not excessive, this excess airrepresents energy which is wasted in the form of overboard release,typically through a modulating outflow valve. In accordance with oneaspect of the present invention this outflow, instead of being dumpedthrough an outflow valve, is directed through the turbine (18). Theturbine (18), forms a part of the energy recovery system (20) of thepresent invention, and mechanically drives the compressor (26). Theturbine (18) and compressor (26) are shown in FIGS. 1-3 as a singlehousing, turbo-compressor unit (30).

One key feature of the energy recovery system (20) lies in theutilization of what would normally be wasted overboard discharge air. Bypurposely recirculating or re-directing the outflow of this discharge ordump air, energy is recoverable via a turbine wheel in the form ofrotational energy.

The compressor (26) is supplied with ram air, which is heated by thecompressor and passed back into the aircraft cabin (16) via the heatexchanger bypass valve (28). If the air is excessively heated by thecompressor (26) it may optionally be passed through the heat exchange(24) for the necessary cooling.

One of the benefits of the use of the turbine (18) lies in the factthat, in expanding the dump air supplied to it, the turbine (18) alsocools this air. Thus, by passing the air exiting from the turbine (18)through the heat exchanger (24), prior to discharging it overboard, itcan be utilized to cool the heated air supplied by the compressor (26).As stated above, if the air supplied by the compressor (26) is of adesired temperature, or if cabin heating is required, the heat exchanger(24) can be bypassed via the bypass valve (28), so that heatedpressurized air can be fed directly to the aircraft cabin (16).

Whenever it is not desired to utilize the energy recovery system (20),discharge air may bypass the turbine (18) via operation of the turbinebypass valve (22). When the recovery system (20) is not operating duringflight, it might not be desirable to back drive the compressor (26). Inthis instance the supply of ram air to the compressor (26) can be cutoff. The bypass valve (32) can also be operated to bypass the compressor(26), allowing for ram air augmentation, that is, outside air can beutilized and brought right into the aircraft cabin (16) either duringlow altitude flight or for ground ventilation. If this outside air istoo hot, the engine bleed air and ECS can be operated, or the outsideair can be brought into the ECS pack (14) for cooling.

The fresh air contribution from the compressor (26), which is suppliedwith outside ram air, is typically a function of the size of theaircraft. Similarly, the quantity of air normally dumped overboard is afunction of the aircraft size and the number of passengers. In a typicallarge aircraft, as much as 210 ppm (3.5 pps) of air may be dischargedoverboard. It is this energy that is used for driving the air turbinemotor (18). Typically, at 35,000 ft., with a 6,000 ft cabin pressure,the airflow/pressure ratio could produce approximately 150 horsepower atthe turbine shaft. If the pressure ratio of the compressor is say 2.6:l(to provide a duct pressure of 12.5 psia at 35,000 ft.), theturbo-compressor unit (30) could provide an airflow of approximately 2.6pounds per second (156 ppm) of "make-up" fresh air. In such a largeaircraft, where the fresh (bleed) air supply might be say 5.0 pps (300ppm), the make-up or energy recovery system (20) could thus provideapproximately 52% of the total fresh air requirement.

The differential in pressure between the interior of the aircraft cabin(16) and the outside depends, of course, on the altitude of theaircraft. It should be apparent that the energy recovery system (20) cansave the most energy at the higher altitudes, such as, for example atcruise altitudes of say about 39,000 feet. Such altitudes, are ofcourse, where the great majority of the longer flights takes place. Itshould be evident also that the energy recovery system (20) isfortuitously applicable at the height altitudes where the reduced airdensity poses problems of maintaining the required air mass flow for thecabin (16).

It is, of course, necessary that bleed air make up whatever shortfall inair requirements occur when the energy recovery system (20) is inoperation. To this end, mass air flow sensors (34) and (38) areutilized. The desired mass air flow into the aircraft cabin (16) at anypoint in time is known, and the mass flow sensor (38) continuouslymonitors this flow, providing a signal, such as an electric analogsignal representative of that value, to the Flow Control Valve (FCV)Control unit (40). In like manner the mass flow sensor (34) monitors themass air flow emanating from the compressor (26), sending a signalrepresentative of the value thereof to the flow control valve controlunit (40). The flow control unit (40) continuously monitors, in knownmanner, the differences between the required cabin mass flow and themakeup fresh-air mass flow, so as to control the motorized flow controlvalve (36) in response thereto, and provide the required engine bleedair input.

FIG. 2 shows a second embodiment of the present invention, wherein theenergy derived from the turbo-compressor unit (30), is used tosupercharge the inlet of a motor driven compressor (42). The compressor(42) is shown driven by an electric motor (44), but it should beunderstood that the cabin compressor (42) could be mechanically drivenby the aircraft engine or any other suitable means. The cabin compressor(42) is used to provide the fresh air required by the aircraft cabin(16), in lieu of engine bleed air. The advantages of the electric motordriven system of FIG. 2 are enumerated in the aforementioned copendingU.S. patent applications, U.S. Ser. No. 183,079, now abandoned, and U.S.Ser. No. 183,609.

As shown in FIG. 2, the compressor (26) is in series with themotor-driven cabin compressor (42), and so the pressure-ratio developedacross the cabin compressor (42) can be reduced by the magnitude of thepressure-rise developed across the compressor (26). Again, assuming 150hp is available to the compressor (26), the possible pressure-rise wouldbe a function of the required cabin air flow. If the desired designcabin air flow is 5 pps (300 ppm), then a compressor (26) pressure-ratioof approximately 1.75 is possible. If the dynamic pressure-inlet tocompressor (26) is, for example, 4.5 psia, then the "supercharge" intothe cabin compressor is 7.8 psia; therefore if a duct-pressure of 12.6psia is required, then the pressure/ratio of the cabin compressor (42)need only be 1.6:1. The cabin compressor could then only require 160 hpto drive it, compared to 280 hp if it had to develop a 2.8:1 pressureratio.

Another benefit of the pressure-boost system of FIG. 2 is that it can becut-out at lower altitudes, when higher air densities andambient-pressures would create overloading of the motor (44). Typically,the turbo-compressor (30) could be cut-out below, for example, 15,000 ftaltitude and the cabin compressor (42) could then supply thepressurization needs by itself. In a non-boosted (conventional) system,high pressure-losses, and low efficiencies, are incident upon the lowaltitude operating condition. To improve the thermodynamic cycle andefficiency of the system described herein, cold air from the output ofthe turbine (18) drive is passed through the heat exchanger (24), priorto dumping overboard.

Since the compressors (26) and (42) are in series, a balance must bemaintained within the ECS so that the aircraft cabin (16) is alwaysprovided with air at the desired pressure at any point in time. To thatend, the pressure sensors and control transducers (48) and (49) areprovided so as to continuously monitor the air pressure emanating fromthe compressor (26) and entering the cabin (16), respectively. Thesesensors provide a signal, such as an electric analog signalrepresentative of the pressure values at those points, to the inletguide van control circuit (40'). The guide van control circuit (40')continuously monitors in known fashion, the differences between thedesign or desired cabin pressure and the pressure supercharge providedby the compressor (26), and in response thereto, operates the inletguide vane control system (46) of the cabin compressor (42) such thatthe rise in pressure across the compressor (42), plus that acrosscompressor (26), totals the desired cabin inlet pressure. When energyrecovery system (20) is not in operation, ram air can be provided to theinlet of the cabin compressor (42) via operation of the bypass valves(28) and (32).

In another variation of the present invention, as represented in FIG. 3,a "fresh-air make-up"--"energy recovery" system as shown in FIG. 1 isutilized in conjunction with the electric motor driven cabin compressor(42) of FIG. 2. As in FIG. 1, the modified energy recovery system (20")utilizes normally wasted overboard discharge air to provide conditionedair to the aircraft cabin (16). In producing this supply of conditionedair, the requirements of air from the output of the cabin compressor(42) are accordingly reduced. To that end, the mass flow sensors (34)and (38) are utilized, via the inlet guide van control circuit (40'), toadjust the inlet guide vanes (46) of compressor (42), and to produce thedesired cabin compressor output. Thus, as discussed hereinabove withreference to FIG. 1, the "make-up"--"energy recovery" system (20") iscapable of reducing the required power input to the cabin compressor(42) by as much as about 52% of the total aircraft air conditioningrequirement.

FIG. 4 shows yet another embodiment of an "energy recovery" system inaccordance with the present invention, wherein power from the cabin (16)discharge-air can be applied directly to an air turbine motor (19) byopening a valve (21) and closing a dump valve (23). The air turbinemotor (19) is mounted on the same shaft of the electric motor (44) thatdrives the cabin compressor (42). This air turbine motor (19) alsoincorporates guide vanes (47) for flow control, and these are actuatedby a cabin pressure sensor and control transducer (48), which controlsan electronic actuator forming a part of the inlet guide vane control(41). Operation is similar to that described hereinabove in relation toFIG. 2 in that the cabin, or inlet duct, pressure is monitored by thepressure sensor and control transducer (49) which controls the inletguide vanes (46) via the inlet guide vane control (40'). At the sametime, the pressure sensor and control transducer (48), sensing the cabinpressure, controls the guide vanes (47) on the air turbine motor (19).

The power feedback contribution to the motor (44) depends upon theavailable discharge-energy from the cabin: when it is high the guidevanes (47) open, allowing the maximum torque-assist to the motor (44)driving the cabin compressor (42). Since the motor (44) may, typically,be a low-step induction motor, its speed can be relatively constant. Inthat instance, reduction in motor-torque, resulting from the torqueassist from the air turbine motor (19), results only in a slight rise inthe speed of the cabin compressor (42). As the aircraft descends fromits highest operating altitude, the air density and pressure increases,resulting in a change in inlet guide vane angle on the cabin compressor(42). When continued descent increases the density and pressure further,the inlet guide vanes (46) reach their maximum limit. Increases in cabinpressure at this point can be controlled by modulation of the emergencydump-valve (23), but, as in all the other embodiments described herein,the motor (42) speed can be reduced, as by means of pole-changing: atechnology well-known by those versed in the art. When the motor speedis reduced, the guide vanes (46) are again brought into their operatingrange. It is therefore evident and implicit, in the description, thatthe energy-recovery system (in all embodiments herein) is not requiredat the lower altitudes. As state, the point-design of the ECS is set bythe maximum altitude condition, where more advantage is to be gained bythe feedback energy.

There have been disclosed, in accordance with the present invention,several embodiments of a balanced power ECS which regenerate the energyof air typically dumped overboard in an aircraft, and thus wasted. Whilethe energy recovery system of the present invention has been describedwith reference to particular embodiments, it should be understood thatsuch embodiments are merely illustrative, as there are numerousvariations and modifications which may be made by those skilled in theart. For example, it should be readily apparent that if it is determinedthat additional cooling of the makeup air produced by theturbo-compressor unit (30) of FIGS. 1 and 3 is desired, this air may bedelivered to the ECS pack (14) for necessary cooling. Thus, theinvention is to be construed as being limited only by the spirit andscope of the appended claims.

Industrial Application

The cabin air discharge energy recovery systems of the present inventionare useful, for example, in conjunction with "bleed air", "mechanically"and "electrically" driven ECS for aircraft. The recovery systemscomprise a fresh-air "make-up" system, cabin compressor inlet"pressure-boost" system and cabin compressor drive-motor assist systemfor optimizing the efficiency of the ECS by utilizing the energy ofnormally-wasted aircraft cabin discharge air.

We claim:
 1. An aircraft environmental control system for supplying apredetermined amount of conditioned air to an aircraft cabin,comprising:an electric motor; a cabin compressor arranged to be drivenby said electric motor, said cabin compressor having an outlet adaptedand arranged to supply heated and pressurized air to said cabin; asupercharging compressor connected in series between a source of freshair and an inlet of said motor-driven cabin compressor; a free-runningturbine mechanically coupled to said supercharging compressor to form aturbo-compressor arranged and adapted to be driven solely by dischargeair emanating from said cabin, whereby the inlet of said motordrivencabin compressor may be provided with pressurized fresh air when saidcabin is discharging pressurized air overboard.
 2. An environmentalcontrol system as in claim 1 wherein said predetermined amount ofconditioned air exceed the amount of air desired in said aircraft cabinduring flight, said turbo-compressor being arranged so as to be drivenby said excess air as it is discharged from said cabin.
 3. Anenvironmental control system as in claim 2 including a heat exchangerinterposed between said supercharging compressor and said motor-drivencabin compressor.
 4. An environmental control system as in claim 3including means for optionally directing the heated air output of saidsupercharging compressor through said heat exchanger.
 5. Anenvironmental control system as in claim 4 including means for directingsaid discharge air through said heat exchanger after it has passedthrough said turbo-compressor.
 6. An environmental control system as inclaim 2 wherein said turbo-compressor includes means for bypassing saidexcess discharged air.
 7. An environmental control system as in claim 2wherein said motor-driven cabin compressor includes inlet guide vanemeans for controlling the pressure rise across said motor-driven cabincompressor.
 8. An environmental control system as in claim 7 includingcontrol means for monitoring the air pressure at the input to said cabinand for adjusting said inlet guide vane means in response to saidmonitored data, so that the desired input of conditioned air to saidcabin is maintained.
 9. A process for supplying a predetermined amountof conditioned air to an aircraft cabin during flight, comprising thefollowing steps:providing an electric motor; providing a primary cabincompressor; adapting and arranging said cabin compressor to be driven bysaid motor with an inlet and an outlet respectively in communicationwith a source of fresh air and with said cabin; connecting said motor toa continuing supply of electrical power to thereby drive said cabincompressor and provide a supply of heated and pressurized fresh air tosaid cabin; providing a supercharging compressor; connecting saidsupercharging compressor in series between said cabin compressor andsaid source of fresh air; and driving said supercharging compressorsolely with energy extracted from discharge air emanating from saidcabin to thereby reduce the electrical energy required to operate saidmotor.
 10. A process as in claim 9 wherein the amount of conditioned airsupplied to said cabin exceeds the amount of air desired in said cabin,said driving of said supercharging compressor further comprising thesteps of:discharging said excess air from said cabin; providing aturbine adapted to be driven by said discharged air, said superchargingcompressor being arranged so as to be driven by said turbine; supplyingthe inlet of said supercharging compressor with ram air; and drivingsaid turbine with said excess discharged air to drive said superchargingcompressor and supercharge said inlet of said motor-driven cabincompressor.
 11. A process as in claim 10 further including the stepsof:providing a heat exchanger in series between said superchargingcompressor and said motor-driven cabin compressor; passing the outletair from said turbine through said heat exchanger; and cooling thehot-pressurized fresh-air from the outlet of said superchargingcompressor by passing it through said heat exchanger prior to beingintroduced into said inlet of said motor-driven cabin compressor.
 12. Aprocess as in claim 10 further including the steps of:providing means tomonitor the pressure of the output of said supercharging compressor andthe input pressure of conditioned air to said cabin; providing inletguide vanes on said motor-driven cabin compressor; monitoring the valuesof the output pressure of said supercharging compressor and the inputpressure of conditioned air to said cabin; and in response to saidvalues, controlling the pressure rise across said motor-driven cabincompressor by adjusting said inlet vanes such that the desired input ofconditioned air to said cabin is attained.